The present disclosure relates generally to hard-facing for downhole tools and earth-boring drill bits used to drill a borehole for the ultimate recovery of oil, gas, or minerals. More particularly, the invention relates to matrix bit bodies, and hardfacing for downhole tools with improved wear (erosion) resistance and fracture toughness.
An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by rotating the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. With weight applied to the drill string, the rotating drill bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone. The borehole thus created will have a diameter generally equal to the diameter or “gage” of the drill bit.
The cost of drilling a borehole for recovery of hydrocarbons is very high, and is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is affected by the number of times the drill bit must be changed before reaching the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipe, which may be miles long, must be retrieved from the borehole, section by section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. This process, known as a “trip” of the drill string, requires considerable time, effort and expense. Accordingly, it is desirable to employ drill bits that will drill faster and longer. The length of time that a drill bit may be employed before it must be changed depends upon a variety of factors, including the bit's rate of penetration (“ROP”), as well as its durability or ability to maintain a high or acceptable ROP. In turn, ROP and durability are dependent upon a number of factors, including the ability of the bit body to resist abrasion, erosion, and impact loads.
Two predominant types of drill bits are roller cone bits and fixed cutter bits, also known as rotary drag bits. FIG. 1 illustrates a typical fixed cutter (FC) bit 10 for drilling through formations of rock to form a borehole. Bit 10 has a central axis 11 about which bit 10 rotates in the cutting direction. In addition, bit 10 includes a bit body 13 and a bit face 20 formed on the end of the bit body 12 (not visible in this perspective) that faces the formation. Bit body 10 can be formed from steel or from a composite material referred to as “matrix.” A cutting structure 21 is provided on the bit face 20 and includes six blades 30 integrally formed as part of, and extending from, bit body 12 and bit face 20. Each blade 30 has a formation facing cutter-supporting surface 31 for mounting a plurality of cutter elements 32 thereto. Each cutter element 32 has a cutting face 44 attached to an elongated and generally cylindrical support member or substrate that is received and secured in a pocket formed in the corresponding surface 31. A plurality of gage pads 40 are disposed about the circumference of bit 10 at angularly spaced locations. Each gage pad 40 is integrally formed as part of the bit body 12 and extends from one blade 30. The radially outer gage pads 40 abuts the borehole sidewall during drilling to help maintain the size of the borehole and stabilize bit 10 against vibration.
Bit performance is often limited by selective/localized erosive damage to the bit body. In FIG. 1, localized regions that typically experience erosion and wear during drilling operations are shown shaded on bit 10, such as is experienced for example at the side wall of the blade. Excessive erosion and wear in such regions can alter and negatively affect specific design parameters for optimal cutting and hydraulic flow paths. For example, excessive localized erosion and wear can alter cutter exposure (i.e., extension height of cutter elements). In addition, excessive erosion and wear around cutter elements (e.g., cutter elements 32) can increase the likelihood of such cutter elements being broken off or otherwise removed from the bit during drilling operations.
To improve the wear resistance of steel bit bodies, a protective coating, often referred to as hard-facing, can be applied to the base metal (steel) of the bit body. The hard-facing is a harder material than the base metal, and thus, enhances abrasion resistance. The durability, and hence effectiveness, of hard-facing applied to a steel bit body is dependent on the coating integrity. In particular, coating failure and exposure of the steel body can lead to accelerated erosive or wear damage effecting bit performance and dull condition of bit.
The propensity of steel body bits to experience erosive damage when in service has been a primary reason for the use of matrix bit bodies for fixed cutter bits. Such matrix bit bodies typically are formed by integrally bonding or welding to a steel blank in a hard particulate (or hardphase) material volume, such as particles of WC (tungsten carbide), WC/W2C (cast carbide) or mixtures of both, and infiltrating the hardphase with a infiltrant binder (or infiltrant), and forming a composite matrix bit body. The composite matrix bit body is removed from the mold and secured to a steel shank having a threaded end adapter to mate with the end of the drill string. PDC cutters are then bonded to the face of the bit in pockets that were cast.
Cast carbide pellets (WC/W2C eutectic) formed from spherical or angular particles (macrostructure) are commonly used in hard-facing for drill bits and downhole tools, whereas cast carbide pellets (WC/W2C eutectic) formed from non-spherical particles (macrostructure) are often added to a matrix bit body for ease of infiltration. However, some degree of dissolution of WC/W2C pellets into Ni-, Co-, or Fe-alloy matrix of the hard-facing is observed, which leads to an increase in matrix hardness and can cause cracking in the hard-facing. For non-spherical cast carbide, at infiltration temperatures of greater than 2000° F., WC/W2C particles can be completely dissolve into Cu—Ni—Mn—Zn alloy of bit matrix and degrade its mechanical properties, leading to a lower fracture toughness for the bit body. The temperatures experienced down hole can often exceed the 2000° F. threshold, potentially resulting in unreliable performance, and in some cases, failure of the hard-facing and matrix bit bodies.